1. Field of the Invention
This invention relates to multiphase core//shell polymers and more particularly to multiphase core//shell polymers containing a rigid shell comprising styrene and a monoalkyl maleate or fumarate, used to enhance the impact resistance and ductility of polyamides.
2. Description of the Prior Art
There is much prior art concerned with improving the impact strength of polyamides. A variety of additives have been added to polyamides with some improvement in toughness being obtained. Many of the additives are elastomeric in nature. For example, Owens et al. U.S. Pat. No. 3,668,274 teaches modestly improved impact strength of polyamides modified with (A) a first cross-linked elastomer phase of copolymers or terpolymers and (B) a final rigid phase thermoplastic stage containing amine-reactive moieties, preferably carboxylic acid groups. The soft modifier is coated with a rigid layer thus apparently negating a large improvement in polyamide toughness.
Dunkelberger, U.S. Pat. No. 4,167,505, recognizes that the polymer modifiers of Owens et al. impart improvement in the impact strength of higher molecular weight polyamides but that the resulting blends do not exhibit the good flow necessary for injection molding operations and teaches that Owens' core//shell polymers having high rubber core content are not capable of being admixed and dispersed in low molecular weight nylon due to the very low viscosity of nylon above the melting point and the resulting difficulty of dispersing a viscous component in a fluid medium due to insufficient shear.
Another approach to the problem of enhancing the toughness of polyamides is provided by Epstein, U.S. Pat. No. 4,174,358. Toughened multiphase polyamides were obtained by incorporating elastomers modified by copolymerization or reaction with monomers containing functional groups such as carboxy groups capable of reaction or hydrogen-bonding with polyamides. This approach has been used with acrylates, polyethylenes, ethylene propylene rubbers, ethylene propylene diene rubbers and ethylene vinyl acetate copolymers. The resulting functionalized bulk rubbers or elastomers require very intensive shear in order to be finely dispersed in a polyamide matrix. The rubbers must, therefore, be soluble (i.e., not crosslinked) in order to permit flow and dispersion on heating. Since the rubber particles are soluble and deformable, their final size is largely dependent on intensity of shear on extrusion and molding. The desired fine rubber dispersions are difficult to obtain without intensive shear, and control of rubber particle size in the final molding is not easily obtained.
Humme et al, U.S. Pat. No. 4,221,879, discloses impact-resistant polyamides consisting substantially of a polyamide and a graft product of a polybutadiene as a graft substrate and a mixture of an acrylate or methacrylate, and acrylonitrile and/or acrylamide monomers grafted thereon. The grafted shell is generally elastomeric in nature. Graft products containing a rigid shell such as a styrene copolymer shell are found to be unsatisfactory apparently because of poor compatibility with the polyamide.